1. Field of the Invention
The present invention relates to an image forming system, a sheet finisher, and a folding method.
2. Description of the Related Art
What is called saddle-stitched or center-folded booklet production has been conventionally performed. The saddle-stitched booklet production is performed by saddle stitching a sheet batch, which is a stack of a plurality of sheets delivered from an image forming apparatus, and folding the thus-saddle-stitched sheet batch in the middle of the sheet batch. Folding such a sheet batch containing a plurality of sheets can cause outside sheets of the sheet batch to be stretched at a fold line by a greater amount than inside sheets. Image portions at the fold line on outside sheets can thus be stretched, resulting in damage, such as come off of toner, to the image portions in some cases. A similar phenomenon can occur when other fold, such as z-fold or tri-fold, is performed. A sheet batch can be folded insufficiently depending on the thickness of the sheet batch.
Creasing devices that produces a crease (scoring) in a sheet batch prior to a folding process where the sheet batch undergoes single fold or the like so that even outside sheets can be readily folded and thereby preventing come off of toner have already been known. Such creasing devices typically produce a crease in a sheet in a direction perpendicular to a sheet conveying direction by moving a roller on a sheet, burning a sheet with a laser beam, pressing a creasing blade against a sheet, or a like method.
An example of such a creasing device is disclosed in Japanese Patent Laid-open Application No. 2008-081258. A technique that allows a roller for use in forming a crease to be interchangeable with an optimum roller suited for paper to be creased so that a crease of a favorable shape and high accuracy can be produced according to a type of the paper is disclosed in Japanese Patent Laid-open Application No. 2008-081258.
Meanwhile, a crease is useless unless the crease is produced on the same position as the position of a fold, which is to be formed through folding subsequent to creasing. Hence, it is required to cause a crease (score) position and a fold position to coincide with each other. If the crease position fails to coincide with the fold position, a double crease or an unfavorable result that sheet edges on the side opposite from the crease are not flush can occur. Aside from this, a crease and a fold are formed at a desired position in a sheet in some cases. More specifically, to perform half fold or center fold (saddle stitching), for instance, it is required to cause a fold position and a stitch position to coincide with each other at a center portion of a sheet in a longitudinal direction.
FIG. 27 and FIG. 28 are explanatory diagrams illustrating an example where a fold position, a crease (score) position, and an intended position (a position where a sheet is to be folded) depart from one another. If, as illustrated in FIG. 27, although it is desired to fold a sheet of paper (hereinafter, “sheet”) P at an intended position (a center position in a sheet conveying direction) F1, a crease is produced at a crease position F2 and a actual fold is actually formed at a fold position F3, and the sheet P is folded at the fold position F3, the crease position F2 departs from the intended position F1 as illustrated in FIG. 28. Furthermore, when the sheet having a left edge FL and a right edge FR as illustrated in FIG. 27 is folded, the overlaid edges fail to be flush with each other, resulting in an edge misalignment FX, which is a misalignment between the left edge FL and the right edge FR.
Such an edge misalignment makes not only creasing useless but also causes an unnecessary crease to be left. To avoid this, when performing creasing, the following adjustments are preferably performed.
1) Causing a crease position and a fold position to coincide with each other.
2) Causing creasing and folding to be performed at a predetermined position in a sheet.
Meanwhile, in a situation where a sheet undergoes post-processing operations, sheet-position adjustment is preferably performed because positions where the post-processing operations are performed typically depart from each other. To perform the adjustment 1) and the adjustment 2) mentioned above, a unit for adjusting the crease position and a unit for adjusting the fold position are required, respectively. However, it is burdensome and less efficient to perform the adjustment 1) to cause the crease position and the fold position to coincide with each other and additionally perform the adjustment 2) to adjust the crease position and the fold position independently.